A device including a pair of seal bars which operate in box motion to grip and heat-seal a tube film covering a number of articles horizontally moving at equal intervals is disclosed in U.S. Pat. No. 5,367,859 and U.S. Pat. No. 5,271,210. As shown in these known examples, the gripping pressure with which the pair of seal bars grip a thermoplastic film is obtained from a mechanical element referred to as a rotation type motor.
To heat-seal a thermoplastic film in a satisfactory manner, the time during which the pair of seal bars grip the film, the calorific power for heating the film, and the gripping pressure on the film have to be properly coordinated. However, as considered from the standpoint of efficiency, the time during which the seal bars grip the film has to be almost an instant and hence the calorific power to heat the film has to be correspondingly set at a higher value.
Further, as can be seen from the above-mentioned known examples, the gripping pressure of the seal bars using a motor as a drive source is constant so long as the operator adds no adjustment thereto. Thus, if the calorific power for the film changes to a lower value, signs of incomplete heat-seal appear in the film, while, reversely, if it changes to a higher value, a keloid phenomenon appears in the heat-sealed portions.
The heat with which to heat-seal the film is obtained either by constantly feeding electric current to a pipe heater embedded in the seal bar to heat the seal bar itself or by applying impulse current to a heat generating filament stuck to the seal bar. However, the heat to heat-seal said film is unstable all the time owing to changes in electric current.
That is, said current is unstable owing to changes in the resistance of heat generating filaments or Nichrome wires. And said current is also unstable immediately after the start of operation of the packaging machine. As a result, the aforesaid incomplete heat-seal or keloid phenomenon appears in the sealed portions of the film.